“High frequency” is the term normally used when the signal propagation delay of a signal from point A to point B on a conductor path is close to the duration of the period of the signal. Frequencies of 3 MHz or higher can be considered high frequency. At high frequencies, conductor path lengths and characteristics of the conductors play a significant role. At high frequencies, it generally cannot be assumed that a signal occurring at a first position along a conductor also occurs at the same time at a second position along the conductor, where the second position is a few centimeters away from the first position and connected to the first position by a conductor path. Generally, the signal does not reach the second position until a few nanoseconds after reaching the first position, and the signal has possibly changed in value when the signal reaches the second position. Thus, for high frequencies (e.g., frequencies of 3 MHz or higher) appropriate technologies are used. Known technologies designed for use with lower frequencies generally are not directly usable for high frequency applications.
For the excitation of plasma processes using high frequencies (e.g., for RF-sputtering, etching or for excitation of gas lasers), other suitable devices along with linear amplifiers (such as Class A and B amplifiers) are primarily HF generators including one or more switching elements. The switching elements can be Class D or Class E amplifiers. In Class E amplifiers, the voltages on the switching elements (e.g., transistors) can increase to over three times that of the DC-supply voltages while the Class D amplifier formed from a bridge limits the voltages on the transistors to the supply voltage.
In the Class D amplifier, two switching elements connected in series, e.g. MOSFETS, are often used. This amplifier arrangement can be referred to as a half-bridge. The usual circuit for a half-bridge is arranged as follows. The higher transistor or switch at the positive DC supply voltage (High Side Switch, HSS) is connected with a drain terminal to the positive DC supply voltage (+V) and the source terminal of the higher transistor is connected to the drain terminal of the lower transistor or switch (Low Side Switch, LSS). The source terminal of the lower transistor is connected to the negative DC-supply voltage (−V). The output signal of the half-bridge is tapped between the two switching elements (e.g., the output signal is tapped at the source of the higher transistor and the drain of the lower transistor). Both the lower and the higher transistors are driven via their respective gate terminals (control terminal).
The lower and higher transistors can be a MOSFET driver hybrid DRF 1200 from the Advanced Power Technology company is a MOSFET with an integrated driver component. A half-bridge with bypass capacitor is described in U.S. Pat. No. 7,161,818.